I have a battery charger with an output rated at 8.5V DC / 1.8A – it's charging four of these:

http://www.dealextreme.com/details.dx/sku.5790

If I tried to charge them with a charger rated at 8V DC / 0.5A – what would happen ?

I assume you're charging a pair in series?

8V wouldn't be able to charge a pair in series to 100% capacity – but would come pretty close – charging voltage is normally 4.2V per cell.

The battery charger MUST be Lithium specific.

4.2V per cell is a very precise voltage. 4.1 is not enough for a full charge and 4.3 is too much. Also they don't take overcharge, so a decent charger will have a timeout facility.

There should be a balancing facility to ensure each battery has an equal voltage across it when charging. If you're making up a pack, the individual batteries should be UNPROTECTED. Then they are connected to a specific circuit tailored for the right number of cells. This circuit then provides the power limiting and voltage balance.

Its more complex than most think!

This made me laugh,

Photo taken just before the caliper went up in smoke?

Cheers Dave – the 'other' charger is not lithium specific (came with a lead acid battery) so I'll not be plugging that in 🙂

Dave- they're shrink-wrapped 🙂

If they are protected cells, then you'll struggle to damage then as the protection circuitry cuts out if you try and over charge them.

The cheapo Li-ion chargers from Trustfire charge a pair in series with no balancing – they (the chargers) also seem to blow up quite frequently as well 🙁

If they are protected cells, then you'll struggle to damage then as the protection circuitry cuts out if you try and over charge them.

If you have a load in series and one is fully charged with the others at, say 75%, then the protection circuit on the first will cut in and prevent the others being fully charged. Result – reduced performance.